Coin operated telephone stations are interfaced with a central office by a two wire loop circuit. When the handset is lifted off the hook, the central office supplies a very small DC loop current, 23 mA minimum, which is available to the coin operated telephone for use as a source of power. When the handset is on the hook, the coin operated telephone draws virtually no current from the phone line. These extreme limits on the power available from the phone line have limited the functions that are incorporated into coin operated telephone stations.
The vast majority of presently existing coin operated telephones or payphones are electromechanical in nature. See, for example, the "Coin Service Booklet" published by New York Telephone. These payphones include an electromechanical coin mechanism for testing the validity and denomination of coins and a limited number of operational functions and features.
With deregulation of the telephone industry, payphones including a number of electronic functions and features have been introduced. These payphones have taken two approaches to address the demand for additional functions and features, and the constraints of low power operation. In some cases, one or more advanced functions have been incorporated by using an internal battery to supply additional power for performing these functions. Other coin operated telephones have simply not incorporated a full range of advanced functions.
While both the desirability and problems of having a coin operated telephone station which operates with very low power, such as the 23 mA available from the phone lines, are well known, the prior art has not successfully achieved such operation in conjunction with providing a full range of advanced functions as described below.